1. Technical Field
The present invention relates to a semiconductor light emitting device, and a planar light source including the semiconductor light emitting device.
2. Description of the Related Art
FIG. 4 shows a surface-mount semiconductor light emitting device 100. As shown in FIG. 4, the surface-mount semiconductor light emitting device 100 includes: a base portion 102; a light emitting diode chip 105; and a resign portion 103. A concave portion is formed in an upper surface of the base portion 102. The light emitting diode chip 105 serves as a light emitting element fixed on a bottom surface of the concave portion by soldering. The resin portion 103 covers the light emitting diode chip 105. The base portion 102 includes a heat radiating plate 106 containing a metal, on which the light emitting diode chip 105 is fixed by soldering, and a surrounding body 104 which surrounds the heat radiating plate 106. In this case, the surrounding body 104 includes a side wall portion 109 of the base portion 102. Lead terminals 107 are led out from a pair of side surfaces of the surrounding body 104 opposite to each other and are electrically connected to the light emitting diode chip 105 via respective wires 108. The combination of the semiconductor light emitting device 100 with a light guide plate configures a planar light source for use in, for example, a backlight for a liquid crystal display. In the planar light source, an upper surface 109a of the side wall portion 109 of the base portion 102 is arranged opposite to a light incident surface of the light guide plate such that light emitted from the light emitting diode 105 enters via the resin portion 103 to the light incident surface of the light guide plate. The light entering to the light guide plate is output from one major surface of the light guide plate to outside of the light guide plate. In order to obtain sharp directivity of light emitted from the semiconductor light emitting device 100 so as to improve luminance of light emitted from the major surface of the light guide plate to the outside, the upper surface 103a of the resin portion 103 covering the light emitting diode chip 105 is positioned substantially flush with the upper surface 109a of the side wall portion 109, or the resin portion 103 protrudes from the upper surface 109a of the side wall portion 109 such that the upper surface 103a is positioned on an upper side of (higher than) the upper surface 109a of the side wall portion 109. JP-A-2007-041471 describes such a planar light source.
In the planar light source, the semiconductor light emitting device is disposed in the vicinity of the light guide plate based upon the below-mentioned reasons:
(1) When a gap or a cavity (hereinafter referred to as “cavity portion”) is provided between the semiconductor light emitting device and the light guide plate, light is refracted due to a difference between a refractive index of an air layer present in the cavity portion and a refractive index of the resin portion, so that an amount of light entering into the light guide plate is decreased. As a result, it is difficult to obtain desirable luminance of the light output from one major surface of the light guide plate to the external portion.
(2) When the cavity portion is provided between the semiconductor light emitting device and the light guide plate, the surface size of the planar light source becomes large.
However, when the semiconductor light emitting device is arranged in the vicinity of the light guide plate, light emitting performance of the light emitting diode chip may be deteriorated. The reason of the deterioration of the light emitting performance is that, in the semiconductor light emitting device, the resin portion of the semiconductor light emitting device is heated by heat generations of the light emitting diode chip, so that the resin portion may be easily and thermally expanded. When the resin portion of the semiconductor light emitting device is thermally expanded and then the expanded resin portion is brought into contact with the light incident surface of the light guide plate, depression force is applied through the expanded resin portion of the semiconductor light emitting device to the light emitting diode chip, since the light guide plate is harder than the resin portion of the semiconductor light emitting device. As a result, mechanical stresses capable of deteriorating the light emitting performance may be produced in the light emitting diode chip, in addition, bonding wires may be cut or stripped from electrodes.
Also, in case where a plurality of semiconductor light emitting devices are arranged in parallel with the light incident surface of the light guide plate, if degrees of protrusion of upper surfaces of resign portions of the plural semiconductor light emitting devices are fluctuated in manufacturing steps, when the resin portions of the semiconductor light emitting devices are thermally expanded, some semiconductor light emitting devices whose protrusion degrees are relatively small do not contact with the light incident surface of the light guide plate, but another semiconductor light emitting devices whose protrusion degrees are relatively large may contact with the light incident surface of the light guide plate. As a result, at the portions where the semiconductor light emitting devices contact with the light incident surface of the light guide plate, the capacity portion disappears, so that light received by the light guide plate from the semiconductor light emitting devices increases. As a consequence, brightness of the planar light source is fluctuated depending upon locations of the semiconductor light emitting devices. Accordingly, the display grade of the planar light source may be lowered.